Introduced plants threaten biodiversity and ecosystem processes, including carbon (C) and nitrogen (N) cycles, but little is known about the threshold at which such effects occur. We examined the impact of the invasive shrub Amur honeysuckle on soil organic carbon (SOC) and N density at study sites that varied in invasion history. In plots with and without honeysuckle, we measured honeysuckle abundance and size (basal area) and extracted soil cores. SOC and N densities were highest at the site with the longest invasion history and highest invasion intensity (i.e., greatest abundance and basal area of honeysuckle). Basal area of honeysuckle positively affected SOC and N densities likely because of increased litter decomposition and altered microbial communities. Because honeysuckle increases forest net primary productivity (NPP) and SOC, it also may play a role in C sequestration. Our results demonstrate the need to consider the influence of invasion history and intensity when evaluating the potential impact of invasive species.

Nomenclature: Amur honeysuckle; Lonicera maackii (Rupr.) Herder LOMA6.

Management Implications: Amur honeysuckle, an invasive shrub in the eastern United States and Canada, forms dense thickets that negatively affect tree seedlings and ground herbaceous vegetation in eastern deciduous forests. Honeysuckle also is allelopathic but other belowground dynamics remain largely unknown, such as whether the shrub affects C and N cycles, which can in turn affect forest productivity. Unfortunately, little is known about how invasion history (e.g., time since invasion) or invasion intensity (e.g., abundance and size of honeysuckle) affects these processes. We established plots with and without honeysuckle at four study sites that varied in invasion history and measured honeysuckle abundance and size as well as SOC and N density within each plot. SOC, N, and abundance and size (basal area) of honeysuckle were highest in plots invaded by honeysuckle at the site with the longest time since invasion. Basal area of honeysuckle positively affected SOC and N densities at this site and likely contributed to increased soil pH. These effects likely were related to increased decomposition of litter and alteration of microbial communities under honeysuckle. Because the effect of honeysuckle on SOC and N density was greatest in stands with the highest basal area, the alteration in density of SOC and N and in the C : N ratio also could positively or negatively affect native herbs or could intensify invasion by other nonnative herbaceous or woody plants. In addition, because honeysuckle affects soil chemistry and increases forest NPP, it may play a role in C sequestration. Resource managers should be attuned to these effects in areas with high abundance of large honeysuckle shrubs (i.e., those with large basal area) and the longest time since invasion. To maximize the limited resources often available for invasive plant control and to reduce ecosystem-level effects, management intervention should be directed toward stands with those characteristics. Our results demonstrate that knowledge of the history and intensity of invasion are important to fully understand the impact of Amur honeysuckle in native forests, especially with respect to belowground dynamics.

Tall buttercup is an invasive forb that has been reported in all but eight states and one Canadian province. The species has been of concern in Montana where it has invaded over 8,300 ha, and it has been particularly problematic in irrigated hayfield meadows that are used for forage production. This study sought to develop an integrated management strategy to control tall buttercup while maintaining forage production. Research was conducted over 2 yr at flood-irrigated and subirrigated hayfield meadows near Twin Bridges, MT. Treatments were randomly applied in a split-plot design with four replications at both sites. Herbicide treatments occurred at the whole-plot level: nonsprayed, aminopyralid (172 g ai ha⁻¹), aminocyclopyrachlor chlorsulfuron (83 g ai ha⁻¹ 33 g ai ha⁻¹), and dicamba (981 g ai ha⁻¹). Split plots consisted of mowing and fertilization (28 kg N ha⁻¹). All herbicides provided up to 2 yr of tall buttercup control at both sites. In the second year, aminocyclopyrachlor chlorsulfuron and aminopyralid reduced tall buttercup by 93% and 96%, respectively, for the subirrigated and flood-irrigated sites. At the subirrigated site, mowing reduced tall buttercup by 71%, and fertilization reduced it by 57%. Forage decreased following aminocyclopyrachlor chlorsulfuron treatments. The integration of herbicide, mowing, and fertilization did not improve tall buttercup control.

Nomenclature: Aminocyclopyrachlor; aminopyralid; chlorsulfuron; dicamba; tall buttercup, Ranunculus acris L. RANAC.

Management Implications: Tall buttercup has been a problematic invader in New Zealand for many years. In North America the species has recently been of concern, especially in Montana where it is believed to reduce forage production in hayfield meadows. However, there are no published studies on tall buttercup management in North America, and landowners have little guidance about how to control the species, while maintaining productivity and plant diversity. In this study, we sought to develop an integrated management strategy to control tall buttercup while encouraging the growth of desirable forage plants, including perennial grasses and clovers. We used two study sites, one in a subirrigated hayfield and a second in a flood-irrigated hayfield in western Montana. We tested four herbicide treatments, including nonsprayed, aminopyralid (172 g ai ha⁻¹), aminocyclopyrachlor chlorsulfuron (83 g ai ha⁻¹ 33 g ai ha⁻¹), and dicamba (981 g ai ha⁻¹), mowing, and fertilization (28 kg N ha⁻¹), applied individually and in all possible combinations. Aminopyralid and aminocyclopyrachlor chlorsulfuron most effectively reduced tall buttercup biomass. Dicamba also reduced tall buttercup compared to the nonsprayed treatment, but was not as effective as the other two herbicides. At the subirrigated site, mowing and fertilization used individually reduced tall buttercup. However, mowing or fertilization at the flood-irrigated site did not reduce tall buttercup, and their combination did not provide control at either site. Moreover, the integration of herbicides with mowing and fertilization did not improve tall buttercup control. Aminocyclopyrachlor chlorsulfuron reduced perennial grasses at both sites, and all herbicides reduced clover, an important forage component of hayfield meadows, compared to nonsprayed plots across both sites. Although combinations of mowing and fertilization with or without herbicides did not improve control of tall buttercup over the use of herbicides alone, long-term integrated management can reduce selection pressure that could lead to herbicide resistance.

Napiergrass has potential as a cellulosic biofuel crop because of its rapid growth habit in the southern United States. However, it is also listed as a potential invasive species by the Florida Exotic Pest Plant Council. For field renovation, information about napiergrass control in response to tillage and herbicides is required. Field studies were initiated to evaluate control of napiergrass established in fields for over 3 yr at Plains, GA, and Tifton, GA. For tillage and POST herbicides, imazapyr plus glyphosate consistently controlled napiergrass relative to diclosulam plus glyphosate, sulfentrazone plus glyphosate, or tillage in terms of visual injury, stem height and dry biomass reduction. One application of imazapyr plus glyphosate controlled napiergrass 74 and 94%, and reduced plant stem height to 6 and 15% of the nontreated control. When diclosulam plus glyphosate, sulfentrazone plus glyphosate, or tillage was used alone with no sequential herbicides, napiergrass control ranged from 12 to 33%; when these control tactics were followed by two sequential applications of either sethoxydim or glyphosate, napiergrass control varied from 45 to 99%. Reductions in plant heights were reflective of injury 47 d after final herbicide applications (May/June). Napiergrass yield in dry biomass production was reduced by imazapyr plus glyphosate ≥ 86% relative to the nontreated control (NTC). Diclosulam plus glyphosate, sulfentrazone plus glyphosate, or tillage alone was not effective in reducing napiergrass dry biomass yields ranging from 1 to 47% compared with the NTC; when these treatments were followed by sequential applications of sethoxydim or glyphosate, napiergrass dry biomass was reduced 46 to 91% compared with the NTC. Tillage plus two applications of sethoxydim or glyphosate exhibited control potential because they provided levels of napiergrass control similar to imazapyr-based treatments. Tillage plus multiple applications of sethoxydim or glyphosate offers flexibility to crop rotations as compared with the residual herbicide imazapyr, which has many crop rotation restrictions because of carryover concerns.

Nomenclature: Diclosulam; glyphosate; imazapyr; sethoxydim; sulfentrazone; napiergrass, Pennisetum purpureum Schumach.

Management Implications: Napiergrass is renowned as having the greatest biomass productivity among herbaceous plants, with estimates of 45 Mg ha⁻¹ yr⁻¹ for potential biofuel production. Although productive as a biofuel crop, elimination of this species from established populations for crop rotation, or controlling escapes that could be invasive, is a concern. Napiergrass can displace native species and alter community structure affecting ecological functions in many ecosystems. Herbicides or tillage or both are potential elimination methods of napiergrass populations from fields, roadsides, and natural areas. Our research indicated that napiergrass control could be achieved by combinations of multiple applications of the sethoxydim, glyphosate, or imazapyr, in combination with moldboard tillage of culms. To achieve complete control in Georgia, three herbicide applications were required, imazapyr was effective alone, tillage was only effective when used in combination with sethoxydim and glyphosate. These data indicate that napiergrass can be controlled. Because of the invasive nature of napiergrass, multiple control tactics will be required for eradication. In southern climates, such as Florida, where moderate winter temperatures are conducive to napiergrass survival, more-intensive management practices with herbicide and tillage may be required for eradication.

Techniques for preventing crazyweed toxicity in livestock have generally fallen into two categories: excluding livestock access to infested ranges during early spring and fall, and controlling crazyweed populations through herbicide application. Although picloram has been used to control crazyweed effectively in the past, aminopyralid has shown efficacy at lower application rates, exhibits less potential off-target movement, and has been classified as a reduced-risk product. Differences in the response of silky crazyweed and nontarget grasses and forbs to picloram 2,4-D and aminopyralid 2,4-D were investigated. Picloram 2,4-D was applied at a rate of 0.3 kg ae ha⁻¹ picloram 1.1 kg ae ha⁻¹ 2,4-D, and aminopyralid 2,4-D was applied at a rate of 0.1 kg ae ha⁻¹ aminopyralid 1.2 kg ae ha⁻¹ 2,4-D. Silky crazyweed canopy cover, number of flowering stalks, plant size, and biomass decreased 15 mo after herbicide treatments (MAT) with average percentage of relative reductions of 92, 95, 90, and 99%, respectively. Crazyweed density decreased by 1.5 ± 0.2 SE plants m⁻² and 1.3 ± 0.2 plants m⁻², a relative reduction of 95 and 80%, 15 MAT in aminopyralid 2,4-D– and picloram 2,4-D–treated plots, respectively. Plots treated with aminopyralid 2,4-D had 4% lower nontarget forb canopy cover than did picloram 2,4-D plots 15 MAT. Grass biomass remained similar within treatments over time for control, aminopyralid 2,4-D and picloram 2,4-D plots, and was similar in all plots 15 MAT. Plots treated with herbicides had, on average, 11% greater grass cover than did control plots 15 MAT (aminopyralid 2,4-D: 89%; picloram 2,4-D: 85%; control: 76%).

Nomenclature: 2,4-D; aminopyralid; picloram; crazyweed, Oxytropis spp., silky crazyweed, Oxytropis sericea Nutt.

Management Implications: Silky crazyweed is a widely distributed and economically damaging poisonous plant that contains the toxin swainsonine. When ingested, swainsonine causes emaciation, abortion, and birth defects in domesticated livestock as well as wildlife species. Typical toxicity prevention methods include limiting livestock access to infested ranges during early spring and fall and controlling silky crazyweed populations through herbicide application. Picloram 2,4-D has been consistently effective in killing silky crazyweed, but a new herbicide, aminopyralid 2,4-D, may be a better option because it is effective at lower rates, has a shorter average half-life, and has less potential for off-target movement. The effects of these two herbicides on silky crazyweed and nontarget forbs and grasses were compared at three sites in northern New Mexico. Picloram 2,4-D and aminopyralid 2,4-D had similar effects on silky crazyweed density, canopy cover, number of flowering stalks, plant size, and biomass. Nontarget vegetation also responded similarly to the two herbicides with grass biomass, canopy cover, and bare ground all having similar values following treatment. Forb canopy cover was lower in aminopyralid-treated plots than it was in picloram-treated plots, suggesting less selectivity with aminopyralid. This could have major implications for important wildlife forage species, but may be beneficial when multiple undesirable forb species are present. Overall, both herbicides effectively controlled crazyweed and increased grass cover.

This paper describes postrelease monitoring of a population of Jaapiella ivannikovi, a gall-forming midge that was introduced for biological control of Russian knapweed. In 2011 to 2013, from late May to early June through August, we monitored 100 permanent plots at one of the first release sites of J. ivannikovi in central Wyoming. Based on the phenology of gall formation, an appropriate window for collection of galls to distribute to new sites is from early to mid-June through early August. Although J. ivannikovi established successfully, 4 yr after release, the percentage of ramets that were galled remained low (1 to 2%), indicating that J. ivannikovi is not yet having a significant effect on Russian knapweed at the site.

Nomenclature: Russian knapweed, Acroptilon repens (L.) DC., ACRE3.

Management Implications: A new biological control agent for Russian knapweed, the gall forming midge Jaapiella ivannikovi was permitted and first released in the U.S.A. in 2009. We addressed two questions of interest to weed managers utilizing J. ivannikovi for Russian knapweed management. What is the appropriate time to collect galls for release at new sites? Is J. ivannikovi having an impact on Russian knapweed four years after release? From 2011-2013, we monitored Russian knapweed ramets (annually) and J. ivannikovi galls (weekly). We also determined emergence times of adult midges by caging galls.

Gall formation occurred from late-May or early-June to mid-August and peaked in early July in 2011 and 2013; there was no clear peak in 2012. Adult midges emerged most frequently from galls between two and three weeks after a gall was first observed. Our results therefore suggest that an appropriate window for collection of galls for release at new sites is from early- to mid-June, about two weeks after galls first appear, through early-August.

Across the three years of monitoring, J. ivannikovi populations were relatively low. Russian knapweed ramet (main shoot) densities were relatively constant across years, and most (98%) Russian knapweed ramets escaped attack by J. ivannikovi. Our results suggest that J. ivannikovi’s impacts, if they do ultimately occur, are likely to take longer than the four years that J. ivannikovi has been present at our site.

Wild parsnip is an invasive species with a global distribution in temperate climates. Parsnips are native to Eurasia and have been cultivated for more than five centuries. It is unclear whether the global invasion of this species is a consequence of escape from cultivation or the accidental introduction of a Eurasian wild subspecies. In this study, we used nuclear ribosomal DNA internal transcribed spacer (ITS) and chloroplast DNA (cpDNA) markers to evaluate the genetic structure of wild parsnip in its native range (Europe) and in three distinct geographic regions where it is considered invasive: eastern North America, western North America, and New Zealand. We also compared wild and cultivated parsnips to determine whether they are genetically distinct. From 112 individuals, we recovered 14 ITS and 27 cpDNA haplotypes. One ITS haplotype was widespread; few haplotypes were rare singletons. In contrast, at least two lineages of cpDNA haplotypes were recovered, with several novel haplotypes restricted to Europe. Cultivated parsnips were not genetically distinct from wild parsnips, and numerous wild parsnip populations shared haplotypes with cultivars. High genetic diversity was recovered in all three regions, suggesting multiple introductions.

Nomenclature: Wild parsnip, Pastinaca sativa L. PASA2.

Management Implications: The availability of molecular data and advances in population genetic analysis have made it possible to estimate the patterns of species migrations with considerable precision. Findings from historical colonization and dispersal studies can shed light on human-mediated transport of biological materials and provide a broad understanding of how humans may have contributed to the introduction and spread of an invasive species. Evidence obtained in this study indicates that escape from cultivation led to the globalization of wild parsnips and introduced populations have high genetic diversity. Increased levels of diversity in invasive populations can act as a primer for rapid adaptive evolution of this noxious invasive species. Future studies with increased sampling and additional loci have the potential to elucidate specific patterns of colonization and admixture in this agriculturally entwined invasive plant and may serve as a model for investigating the natural and agricultural history of plant species that exist in wild, cultivated, and feral forms.

Many conservation land managers working with invasive plants rely largely on their own experience and advice from fellow managers for controlling weeds, and rarely take into consideration the scientific literature, a concrete example of a knowing–doing gap. We argue that invasion scientists should directly teach managers best practices for control. In 2013, we created a training program on five invasive plant species, specifically tailored to Québec (Canada) environmental managers. The course material was science-based, and included details on methods and costs. Here, we explain how this idea emerged, how the program was constructed and which types of managers were targeted. With modest resources, we reached 163 managers in less than 18 mo, who collectively oversee invasive species management for 41% of the Québec population. We presented factual information for all control methods, giving the environmental managers the tools to critically and objectively assess various options. Participants especially appreciated the highly practical content of the training and that they could submit their own invasion case for discussion. This program represents significant progress in narrowing the knowing–doing gap associated with the control of invasive plants in Québec, and we encourage such initiatives elsewhere for all fields of invasion biology.

Biological invasions and climate change pose two of the most important challenges facing global biodiversity. Of particular importance are aquatic invasive plants, which have caused extensive economic and environmental impacts by drastically altering native biodiversity and ecosystem services of freshwater wetlands. Here, we used the maximum entropy model, Maxent, to model the potential range expansion of three nonnative aquatic invasive plants: alligatorweed, limnophila, and giant salvinia, throughout the continental United States under current, 2030 to 2059 (2040), and 2070 to 2099 (2080) climate scenarios. Maxent is a popular method to model predicted current and future species distributions based on biogeography and climate. Alligatorweed, limnophila, and giant salvinia are noxious invaders of freshwater habitats in the southeastern United States and cause economic and ecological loss. In addition, we analyzed each species' habitat preference based on wetland type, occurrence in man-made habitats, and distance to the nearest stream to better understand what future habitats are at risk and how these species spread. Our results show that in 2040 and 2080 climate scenarios, all three species have the potential to increase their range throughout the northeastern United States and as far as New York and Massachusetts. The spatial distribution of alligatorweed was primarily determined by precipitation of the warmest quarter (15.8%), limnophila was primarily determined by precipitation of the warmest quarter (52.2%) and mean temperature of the coldest quarter (21.8%), and giant salvinia was primarily determined by the mean temperature of the coldest quarter (24.3%). All three species were found significantly more frequently in lakes and ponds than in other freshwater habits. Giant salvinia was found significantly more often in man-made wetland habitats. In order to reduce the detrimental impacts of these species, land managers in the northeastern United States should concentrate early detection and rapid response management in lakes, ponds and man-made wetland habitats.

Nomenclature: Alligatorweed, Alternanthera philoxeroides (Mart.) Griseb; giant salvinia, Salvinia molesta Mitchell; limnophila, Limnophila sessiliflora (Vahl) Blume.

Management Implications: Aquatic invasive plant species are well known to have dramatic impacts on the habitats they invade. The impacts of these species include decreases in plant and animal biodiversity, altered nutrient cycling, and impact navigation and recreation of inland waterways. Climate change is expected to amplify the number of aquatic biological invasions by changing climatic conditions, particularly by warmer temperatures, increasing the likelihood that previously climatically restricted species will be successful in northern latitudes. Because of the serious consequences aquatic invasive plants pose to aquatic and wetland habitats, understanding the future range expansion of these species is imperative for early detection and management. Alligatorweed, limnophila, and giant salvinia are noxious invaders of freshwater habitats in the southeastern United States. In this study, we ask the following questions: What is the potential for range expansion of three highly invasive plant species in current and future climate scenarios? and What are the aquatic and wetland habitat preferences of these three species? Our results show that with future climate change, and consequently, warmer temperatures, these three species will have the potential to expand their ranges into the mid-Atlantic and northeastern United States. Lakes and ponds are at increased risk for future invasion because all three species were more frequently found in these habitats over other freshwater wetland types. Furthermore, giant salvinia was found significantly more often in man-made wetland habitats, which co